Forming molds having generally curved glass bending surfaces for bending flat glass sheets into useful shapes such as automobile windshields, side windows and rear windows are generally fixed mold or articulated mold types. The fixed mold can be used in a horizontal position where gravity supplies the force that causes heat softened glass on the curved mold to sag to the extent that it comes in contact with and completely conforms to the shape of the bending surface of the mold as the glass surface slides along the curved surface of the mold. As the glass slides down the surface of the mold, the natural abrasion of the glass against the bending surface causes glass marking and mold wear. Fixed molds are also generally used for press bending heated glass sheets. In press bending, the heated glass sheets are placed between male and female mold members and a mechanical force is used to press form the glass between the bending surfaces of the mold members.
Alternatively, the articulated type mold has a hinged curved bending surface. The articulated mold also uses gravity to supply the force to bend the flat glass. This type of mold has particular application in the formation of glass sheets such as automobile windshields where the radius of curvature near the edge of the glass sheet is less than that of the windshield as a whole. Unlike the fixed shape glass forming mold, there is less tendency for the heat softened glass to slide along the surface of the mold and cause marking of the glass sheet and related mold wear that is often created when a fixed mold is used with gravity supplying the bending force. Articulated molds like fixed shape molds are commercially used for forming flat glass sheets and the associated production rates require a long mold life together with the ability to produce a high quality glass product free from marks associated with hot glass sliding along the curved mold surface.
Conventional molds are normally made of iron, steel or ceramics. Some of these molds have been known to be coated with boron nitride, graphite or various fiberglass coverings. One such mold is known to have boron impregnated directly into the surface of the mold. Nevertheless, lives of conventional molds are limited and often coatings on the molds must be continuously replaced. There is thus a need for a glass sheet bending mold that has a long life while also minimizing glass marking even after repeated bending of the glass sheets.
Prior art references noted by a search conducted prior to filing this application are discussed below.
U.S. Pat. No. 2,201,049 to R. W. Moore for Glass Fabrication Process and Mold provides for mold members which are coated with boron nitride to prevent sticking of fluid glass to the solid mold surface.
U.S. Pat. No. 2,246,463 to Louis D. Garratt for Treatment of Mold Surfaces provides a process for depositing a graphite layer on the surface of a forming mold.
U.S. Pat. No. 2,560,599 to J. D. Ryan for Bending Mold and Method of Prolonging the Life Thereof provides for a glass forming mold having a fiberglass cover and filler.
U.S. Pat. No. 2,746,209 to E. L. Walters for Equipment for Bending Glass Sheets calls for a cast iron mold having high percentages of nickel and chromium.
U.S. Pat. No. 3,295,346 to H. B. Bomberger, Jr. for Methods for the Elevated Temperature Production of Metallic Surfaces, and Coatings Therefor discloses a boron oxide and feldspar coating for a metal surface exposed to hot processing.
U.S. Pat. No. 3,506,430 to W. W. Oelke et al for Glass Sheet Bending Mold discloses a fiber metal material covering for bending molds.
U.S. Pat. No. 3,586,493 to G. R. Claassen for Mold for Bending Glass Sheets provides for a refractory insulating coating on a bending mold.
U.S. Pat. No. 3,713,798 to Stilley et al for Method of Press Shaping Glass Sheets discloses a method of bending heat softened glass sheets against a mold member that uses a fiberglass fabric cover.
U.S. Pat. No. 3,899,316 to Ehlers for Bending of Glass Sheets provides a method of bending heated glass sheets against a resilient layer of alumino silicate fibermat material covering a mold surface.
U.S. Pat. No. 3,973,943 to Seymour for Apparatus for Supporting Shaped Glass Sheets for Tempering discloses a forming mold having a fiber reinforced resin material covering.
U.S. Pat. No. 3,994,707 to Newing, Jr. et al for Solid Film Lubricant Laminates and Their Use in Glass Manufacturing discloses a metal mold covering of nickel and chromium and metal carbides and/or transition metal nitrides together with a graphite lubricant layer. One disclosed composition of the covering has 61% Ni and 7% Cr and about 12% by weight of tungsten carbide.
U.S. Pat. No. 4,382,811, to Luscher et al for Method of Producing Protective Coating On Metal Parts to be Used in Contact With Molten Glass presents a method of flame spraying the metal part with ,hromium oxide and a nickel base alloy.
U.S. Pat. No. 4,251,254 to Klomp et al for Tools for the and Shaping of Glass discloses a tool made oi a boron containing material for the handling and shaping of glass at low viscosity.
U.S. Pat. No. 4,274,857 to Wolfe for Treating Covers for Press Bending Molds, and Method of Press Bending Glass Sheets provides a flexible cover of fiberglass fabric impregnated with a dispersion of boron nitride to be used as a cover with a press bending mold.